It is beneficial to estimate the quality of the receiving channel in a radiotelephone system. A radiotelephone system typically contains multiple fixed-site transceivers which are capable of serving multiple radiotelephones. The quality of service between each of the fixed-site transceivers and the particular radiotelephone varies. If the radiotelephone can estimate the quality of the receiving channel, then the radiotelephone system can choose the most appropriate fixed-site transceiver.
Several different conventional methods of estimating the quality of the receiving channel exist. Typically, the estimate of the channel quality includes an estimate of the bit error rate (BER). A particular time division multiple access (TDMA) radiotelephone system divides time into superframes, frames and timeslots as shown in FIG. 13. The system specification requires a channel quality estimation (CQE) for each superframe. Each superframe is referred to as an observation interval. The radiotelephone shall identify one of four possible performance categories, with each category corresponding to a specified range of channel BERs as illustrated in Table 200 of FIG. 2. Moreover, the specification requires that the radiotelephone identify the correct performance category at the standard BERs in 40 Hz flat Rayleigh fading.
The accuracy of conventional channel quality estimation techniques is insufficient because of the limited number of bits within the CQE observation interval in the aforementioned TDMA system requiring a CQE each superframe. "Techniques for Estimating the Bit Error Rate in the Simulation of Digital Communications", IEEE Journal on Selected Areas in Communications, VOL. SAC-2, No. 1, January 1984 may be referenced to determine the proper number of bits to observe for an estimated BER with a desired confidence level.
A superframe in this system includes 36 frames. A frame contains 140 symbols assigned to each radiotelephone. Thus, a radiotelephone receives 5040 symbols every superframe. Each symbol contains two bits. Two conventional CQE techniques include (1) counting bit errors over known portions of the frame (e.g., sync words, preambles, etc.), and (2) re-encoding the decoded data bits and comparing the resulting bitstream against the received channel bits. This second technique operates only on forward error corrected (FEC) bits. Both of these techniques are inappropriate for this particular system because the number of bit errors observed over a superframe would be insufficient to provide the desired accuracy.
CQE accuracy can be improved using soft error information available at the output of the demodulator. Phase error information is a particular type of soft error information. Accumulating the phase error magnitude (or squared phase error magnitude) over the CQE observation interval and comparing the result against a pre-determined set of thresholds would improve the CQE. However, this technique is too sensitive to the channel fading rate and provides different results for static and fading environments. This is a result of the detector error magnitude (or squared error magnitude) being a non-linear function of the channel bit error rate.
An accurate CQE is desirable for use in a TDMA receiver. The CQE should be accurate to a predetermined confidence level in both a fading and static environment for an observation interval having a limited number of observation bits.